Optical Critical Dimension (also termed “optical CD” or “OCD”) measurement techniques (known also as Scatterometry) are known as efficient techniques for measuring parameters of patterned (periodic) structures. Measurement of these parameters provides a viable metrology solution for process control in high volume manufacturing of semiconductor devices.
OCD measurements are usually performed using a fitting procedure. According to this procedure, theoretical model describing a structure under measurements is used to generate theoretical data or reference data, and the latter is iteratively compared to measured data, while varying the model parameters until the “best fit” is found. The parameters of the “best fit” model are considered as corresponding to the measured parameters. Measured data (typically optical data) can be analyzed to derive information regarding the geometrical parameters of patterns including thicknesses, critical dimension (CD), line spacing, line width, wall depth, wall profile, etc., as well as optical constants of materials included in a sample being measured.
Optical metrology tools used for such measurements are typically ellipsometry and/or reflectometry based tools. Reflectometry based tools typically measure changes in the magnitude of radiation, whether un-polarized or polarized returned/transmitted from/through the sample, and ellipsometry based tools typically measure changes of the polarization state of radiation after interacting with the sample. In addition or as alternative to these techniques, angular analysis of light returned (reflected and/or scattered) from a patterned (periodic) structure could be used to measure the parameters that define/characterize the structure.